1. Field of the Invention
The present invention relates to a method of manufacturing sheave members for use in a belt-type continuously variable transmission.
2. Description of the Related Art
The stepped automatic transmission in which various gears are selectively engaged, and what is termed a belt-type continuously variable transmission are examples of speed change apparatus for transferring drive power from the engine of an automobile to the drive wheels. The belt-type continuously variable transmission provides a continuous range of speed change, with a belt running between an input-side pulley and an output-side pulley, by changing the pitch (effective) radius of the pulleys.
Each pulley includes a shaft, a fixed sheave having a conical sheave surface that is fixed to the shaft, and a movable sheave that has a conical sheave surface and that is mounted on the shaft for movement relative to the fixed sheave. The fixed sheave and the movable sheave, for the sake of convenience, are herein referred to simply as CVT sheaves.
These CVT sheaves are provided with a sheave surface for frictional contact with the belt, and higher abrasion resistance of the sheave surface and precision of the sheave dimensions have been actively pursued as quality improvements. Thus, for example, in the conventional manufacturing methods for CVT sheaves, after forming the sheave into the desired shape, the sheave is carburized by a quenching process immediately after prolonged heating. Subsequently, grinding is employed in order to improve the dimensional precision. In addition, after the grinding process, in some cases the sheave surface is subjected to shot peening, as disclosed, for example, in Japanese Patent Application Publication No. JP-A-2000-130527.
However, in the manufacturing of a CVT sheave, what is termed the carburization quenching process, in which quenching is carried out immediately after prolonged carburization, conventionally uses oil quenching which suppresses the occurrence of distortions more than water quenching. However, notwithstanding the reduction in distortion achieved by oil quenching, it is not possible to provide dimensional precision which meets the required quality. Thus, as mentioned above, grinding is employed after quenching. However, use of grinding to correct imperfection in the dimensions and shape caused by distortion, requires a grinding allowance which is comparatively deep. In addition, in the carburization process, it is necessary to form a carburized layer to a depth equal to or greater than the grinding allowance, such that a carburized layer remains even after grinding, and an extremely long carburization heating time is necessary.
In light of the foregoing circumstances, in order to manufacture CVT sheaves more efficiently, reducing the manufacturing time while maintaining dimensional precision and hardness characteristics, has been demanded.